Microencapsulation of pesticides is one method for controlling the release of the pesticide after application, particularly when sustained or slow release of the pesticide is desired. In the case of some herbicides, the release rate needs to be controlled so that crop injury can be managed. For example, in the case of acetamide herbicides, sustained release is desired because injury in certain susceptible crops has been observed with application sprays prepared from conventional emulsifiable concentrate formulations (non-encapsulated herbicide formulations). Also, slower release can beneficially provide longer residual activity for pest control.
To form microcapsules, the pesticide is encapsulated in a polymeric shell wall material. The pesticide is released from the microcapsules at least in part by molecular diffusion through the shell wall. Several factors including the type of pesticide, type of polymer, shell thickness, shell porosity, particle size, and presence of safeners impact the rate of release of the pesticide from the microcapsules. Modification of these factors to increase or decrease pesticide release rate has definite limitations. Also, once the microcapsules are formulated, the release rate is generally fixed and not tunable. Thus, there remains a need for formulations containing microencapsulated pesticides in which the rate of release of the pesticide is modulated or tunable to provide the desired level of pest control and avoid negative impacts such as crop injury in the case of herbicides.
With regard to herbicides, the emergence of certain herbicide resistant weeds has generated interest in developing strategies to supplement the action of primary herbicides such as glyphosate. Acetamide herbicides are known as effective residual control herbicides that reduce early season weed competition. In particular, acetamide herbicides such as acetochlor provide outstanding residual control of many grasses and broadleaf weeds including pigweed, waterhemp, lambsquarters, nightshade, foxtails, among others. Acetamides are generally classified as seedling growth inhibitors. Seedling growth inhibitors are absorbed and translocated in plants from germination to emergence primarily by subsurface emerging shoots and/or seedling roots. Acetamide herbicides typically do not offer significant post-emergence activity, but as a residual herbicide provide control of newly emerging monocots and small-seeded dicot weed species. This supplements the activity of post-emergent herbicides that lack significant residual activity.
Crop injury caused by application of acetamide herbicides necessitated strategies to reduce this effect including microencapsulating the herbicide. Methods for producing microencapsulated acetamides are described in various patents and publications including U.S. Pat. No. 5,925,595; U.S. Publication No. 2004/0137031; and U.S. Publication No. 2010/0248963.
Herbicide compositions containing a combination of herbicides with multiple modes of action that can supplement the action of primary herbicides such as glyphosate are especially suited for controlling growth of unwanted plants, including those with selected herbicide resistance. However, the release properties of herbicidal concentrates of microencapsulated acetamide herbicides can be sensitive to the inclusion of further additives including co-herbicides. Accordingly, there remains a need for highly concentrated herbicidal compositions containing microencapsulated acetamide herbicides and co-herbicides that can be economically produced while maintaining the release properties of the microencapsulated acetamide herbicide and that can be diluted to provide effective spray formulation solutions for application to unwanted plants.